The present invention relates to a catalyst system capable of removing sodium from a hydrocarbon feedstock and a process using this system. It is in general terms a fixed bed catalyst system. More particularly, it comprises a layer of catalyst particles characterized as having a low volume percent of their pore volume in the form of macropores, having a high surface area, and having a high hydrogenation activity. The process which uses this catalyst system comprises passing a sodium-containing hydrocarbon feedstock over the catalyst system at hydrodemetalation conditions.
Most heavy crudes contain significant amounts of organic metal compounds such as nickel and vanadium. Some are present as insoluble salts which can be removed by conventional filtrating and desalting processes. Yet most of them are present as oil-soluble compounds which are not removed and continue on to the catalyst bed. They create problems for refiners by depositing just below the external surface of the catalyst particles. As a result, they block the catalyst pore openings and deactivate the catalyst.
Previous workers in the field have suggested a variety of schemes to remove the oil-soluble nickel and vanadium organometallics from petroleum feedstocks. One approach is frequently to replace the fouled catalyst, but this is wasteful and results in costly under-utilization of the catalyst. In recent years, workers in the field have developed hydrodemetalation (HDM) catalysts to protect the more active hydrodesulfurization, hydrodenitrification, or hydrocracking catalysts. Generally, the HDM catalyst contacts the contaminated feed and the metals are deposited before the feed continues through the catalyst bed contacting the active catalysts. In particular, complicated schemes of grading varieties of catalysts which differ in pore size, support composition, and metals loading can result in more efficient use of the individual catalysts.
Most grading schemes invole contacting the hydrocarbon feedstock with a catalyst having large pores designed for metals capacity followed by catalysts with smaller pores and more catalytic metals to remove sulfur and other organic metals. In this way the contaminated feed initially contacts a less active catalyst, thereby allowing the feed to penetrate the catalyst more fully before metal deposition occurs. As the less contaminated feed continues through the catalyst bed, it contacts more active catalysts which promote the deposition of sulfur and other organic metals. Thus, for any given feedstock containing metals that penetrate to the interior of the catalyst, such as nickel and vanadium, there will be an ideal grading of catalyst which will result in the the most efficient use of these catalysts from the top of the reactor to the bottom.
Conventional processes, which remove nickel, vanadium, and iron, generally have decreasing macroporosity and increasing mesoporosity in the direction of feed flow through the graded bed. The term "macropore" is used in the art and is used herein to mean catalyst pores or channels or openings in the catalyst particles greater than about 1000 .ANG. in diameter. Such pores are generally irregular in shape and pore diameters are used to give only an approximation of the size of the pore openings. The term "mesopore" is used in the art and used herein to mean pores having an opening of less than 1000 .ANG. in diameter. Mesopores are, however, usually within the range of 40-400 .ANG. in diameter.
Previous workers have found macroporosity to be strongly related to the capacity of catalyst particles to retain metals removed from a hydrocarbon feed contaminated with nickel, vanadium, and iron. In the later catalyst zones, they prefer predominantly mesoporous catalysts. They found these catalysts to have substantially higher catalytic activity for hydrogenation compared to catalysts having lower surface areas and substantially a macroporous structure. Thus, they exploited these two phenomena to remove nickel, vanadium, and iron from heavy feedstocks in graded catalyst systems.
We encounter a more difficult problem when sodium is present in the hydrocarbon feedstock. Sodium typically exists as a metal oxide, sulfide, or chloride appearing as a sodium salt. But it can also be present as an oil-soluble organometallic compound, including metal naphthenates. The present invention particularly addresses this complex metal contaminant problem.
We can easily identify and remove oil-insoluble sodium salts by conventional desalting and filtering techniques. But we know the soluble sodium compounds with less certainty. They are difficult to trace and cannot be removed by conventional methods. Moreover, catalyst systems, like those described above, which are effective for the removal of nickel and vanadium are impotent in controlling the deleterious effects of sodium deposition.
Organic sodium compounds deeply penetrate the catalyst particles working to block the active sites within the catalyst and rendering it deactivated. As a result of our work it has become clear that we cannot use conventional graded systems successfully to remove sodium from hydrocarbon feedstocks. Thus, it is necessary for us to devise a catalyst system, taking into consideration such factors as porosity and hydrogenation activity of the catalyst particles, that successfully removes sodium from hydrocarbon feedstocks. Accordingly, it is an object of this invention to provide such a catalyst system.